In railway service, rails are nominally parallel with a known elevation and a known cant with respect to a horizontal plane. Railcar wheelsets are mounted in pairs on a suspending device referred to as a truck (also called a bogie). Minimum wear on components and maximum ride comfort occurs when the wheelsets are centered on the rail with axes of rotation perpendicular to the rail centerline; any deviation from this alignment and orientation introduces vibration and results in increased wear.
Several basic measures of misalignment have been related to reduced component life and ride comfort, including angle-of-attack (AOA), tracking position (TP), shift, inter-axle misalignment, and rotation. A primary measure, AOA, is defined, from a measurement point of view, as the angle between the plane containing the rim face of a railcar wheel and a tangent line to the rail on which the wheel is engaged. TP is defined as the transverse displacement of the centerline of the wheelset from the centerline of the rail pair. Additional derived measurements related to AOA and TP are made to identify particular anomalies that have been correlated to reduced component life and ride comfort. The measurements assess the translational and rotational misalignments between the two axles on a truck, and between the axles and the rails. Finally, hunting is a term describing periodic transverse motion of the railcar on the track that may, in severe cases cause resonant oscillation, which results in the wheel flanges impacting the rail. This condition can result in rapid component wear and serious ride comfort issues. Serious truck geometry errors can even result in derailment, especially when operating at high speed and when cornering, causing considerable damage and potential loss of life. Thus an accurate and timely measurement of truck alignment errors can result in reduced maintenance costs and possible prevention of catastrophic derailments.
In general, two technologies have been applied to measure truck related geometry anomalies. In a first approach, strain gauges are mounted to the rail to measure the vertical and lateral forces. In this approach, the ratio of the lateral force to the vertical force is indicative of wheelset misalignment. Such a system, however, requires expensive and time consuming changes to the track infrastructure. For example, installation of strain gauges on a track typically requires grinding the rail and the placement of concrete sleepers to properly support the section of track for accurate strain measurement. If the instrumented rail sections are changed out, the system functionality will be lost.
In a second approach, a wayside optical system comprising a laser beam and an optical detector in conjunction with a wheel detector is used to make the measurements using the principle of optical triangulation. In this case, a point laser displacement measure device is used, which may measure 10,000 points/sec on the field side rim face of a passing wheel.
Unfortunately, this approach is only robust for new, good-condition wheels. In particular, the laser is typically applied at an elevation of approximately one inch above the rail. For good-condition wheels, this allows a continuously measurable section of rim face of about ten inches (or at 10 k points per second at 60 mph, about 110 points). However, as the wheel wears, the rim face becomes more and more narrow, resulting in two separated measurement regions which become smaller as the wheel continues to wear. For the worst case of a condemnable wheel, only 5 data points will be produced for a train speed of 60 mph. As the corners of the rim face may be contaminated with debris, dirt, snow, ice, or the like, inconsistent measurements may result, especially in the case of the more worn wheels for which the measurements have less redundancy to allow for the elimination of outliers.
Another significant limitation of this approach derives from the fact that the measured points are in a time-sequence along a moving object. As there are modes of movement of the wheels in which the alignment of the wheel will vary throughout a complete revolution, this method of measurement may be confused or at least rendered less accurate through variations in the wheel orientation over time.
In a variant of the second approach, proximity sensors, such as inductive sensors, are attached to the rail to measure the duration and relative timing of the signal generated by the passing wheels. By employing two sensors, one on each rail, the angle of attack and other truck performance parameters may be measured. This approach is sensitive to the diameter, speed, and condition of the surfaces of the wheel at the point of detection. In particular, proximity sensors are known to have response variations to all of these conditions, and any variation in response can result in an incorrect measurement of the target parameters.